Firing circuit for thyristors

ABSTRACT

A control circuit for supplying trains of firing pulses to each of a plurality of thyristors. A plurality of oscillator circuits, having feedback windings coupled to respective outputs, supply firing pulses to respective thyristors. The order in which the different oscillator circuits are caused to produce firing pulses is controlled by a multistate switching circuit. The feedback winding of the energized oscillator circuit disables all other oscillator circuits to prevent simultaneous application of firing pulses to more than one thyristor during the time when the multistate switching circuit is changing states.

United States Patent Inventor Pantelis P. Paradissis Amherst, Ohio Appl. No. 836,487

Filed June 25, 1969 Patented Jan. 26, I971 Assignee Lorain Products Corporation a corporation of Ohio FIRING CIRCUIT FOR TIIYRISTORS 6 Claims, I Drawing Fig.

US. Cl 307/ I06 Int. Cl H03k 3/00 Field ofSearch". 307/106,

I07, I08, I09; 33l/(Consulted); 333/(Consulted) [56] References Cited UNITED STATES PATENTS 2,819,413 1/1958 Clapper 307/106 3,309,540 3/1967 Hubbs 307/l06 Primary Examiner-Robert K. Schaefer Assistant Examiner-H. J. Hohauser Att0rney-J0hn Howard Smith ABSTRACT: A control circuit for supplying trains of firing pulses to each of a plurality of thyristors. A plurality of oscillator circuits, having feedback windings coupled to respective outputs, supply firing pulses to respective thyristors. The order in which the different oscillator circuits are caused to produce firing pulses is controlled by a multistate switching circuit. The feedback winding of the energized oscillator circuit disables all other oscillator circuits to prevent simultaneous application of firing pulses to more than one thyristor during the time when the multistate switching circuit is changing states.

ALLA ALAAA SWITCHING MEANS a ban LAAA IOd PATENIED JANZB I97| I INVENTOR. PANTELIS F! PARADISSIS WMDpM.

1 FIRING CIRCUIT FOR TIIYRISTORS BACKGROUND OF THE INVENTION The .present invention relates to control circuits and is directed more particularly to thyristor control circuits for furnishing coordinated'trains of firing pulses to a plurality of gate controlled-switching devices.

In the firing of gate controlled-switching devices such as thyristors, it has been the practice to utilize a comb or succession of firing pulses. While this has solved some of the problems resulting from the utilization of a single firing pulse, it has also increased the risk of providing simultaneous firing pulses to more than one thyristor. These simultaneous pulses can cause simultaneous conduction in thyristors which are intended to operate only alternately and thereby produce a serious fault condition.

One method of providing a comb of firing pulses to each of a plurality of thyristors has been to provide a plurality of firing pulse-generating circuits, there being a pulse-generating circuit associated with each thyristor/The desired firing pattern of the different thyristors was then imposed by a multistate, multioutput switching circuit which caused the-pulse-generating circuit to become energized in the desired sequence.

An important difficulty in the use of firing circuits of the above type is the possibility of simultaneity in the firing pulses I produced thereby. This may occur during the period when the SUMMARY OF THE INVENTION Accordingly, it is an object of the invention to provide an improved control network for use in circuits of the character which furnish trains of firing pulses to each of a plurality of gate controlled-switching elements.

It is another object of the invention to provide a control circuitwhich includes a plurality of firing pulse-generating circuits, a switching circuit disposed in energizing-deenergizing relationship thereto, and a start suppression circuit, whereby each. firing pulse-generating circuit is prevented from turning on during the time when a pulse-is being produced in any other firing pulse-generating circuit. I Still another object of the invention is to provide a start suppression circuit which utilizes the pulses produced by each firing pulse-generating circuit to reduce the current available for initiating pulse generating activity in all other pulse-generating circuits, thus eliminating simultaneity between the firing pulses of the different firing pulse-generating circuits.

Generally speaking, the invention comprises a start suppressio'n circuit for thyristor control networks of the type which utilize a switching circuit for sequentially energizing each of a plurality of firing pulse-generating circuits which, in turn, furnish firing pulses to respective thyristors. The start suppression circuit includes windings disposed in pulse responsive relationship to the outputs of respective pulse-generating circuits. The winding of each pulse-generating circuit is disposed in disabling relationship to all other such circuits. Thus,

DESCRIPTION OF THE DRAWINGS The single drawing is a schematic diagram of one circuit embodying the invention.

DESCRIPTION OF THE INVENTION Referring to the drawing, there is shown a suitable power circuit 10. Firing signals for the thyristors of power circuit 10 are "furnished by respective firing pulse-generating circuits 11A and 11B. Firing pulse-generating circuits 11A and 11B are energized alternately and severally in response to control signals generated by switching means 12. A start suppression circuit, which in the present instance includes windings 13a and 13b, ensures that firing pulses are not produced simultaneously in firing pulse-generating circuits 11A and 115.

Power circuit 10 comprises an inverterof the well-known McMurray-Bedford type, wherein alternate firing of thyristors 10a and 10b causes an AC voltage to be established across the primary winding We of a transformer 10:! thereby supplying an AC load'l0e from a DC source 10f. The operation of power circuit 10 will not be described in detail since the MeMurray- Bedford type of circuit is well known to those skilled in'thc art and this circuit is intended as merely one example of the type of circuit with which the control circuit of the invention may be advantageously used.

To the end that thyristors 10a and 10b maybe rendered conducting alternately and severally, firing pulse-generating circuits 11A and llB, which here take the form of blocking oscillators, are connected across a source-of operating potential 14 through a positive bus 14a and a ground connection 15a. Firing pulse-generating circuits 11A and 11B impress trains of firing pulses on output windings 16a and 16b, respectively, in accordance with the potential between ground and junctions 17a and 17b, respectively. Thus, junctions 17a and 17b serve as control junctions for firing pulse-generating means 11A and 11B, respectively.

Because of the similarity between oscillator circuits 11A and 1 13, corresponding parts of these circuits are labeled with the same number but differing postscripts, the postscript a being associated with oscillator 11A and the postscript b being associated with oscillator 11B. Thus, it will be understood that remarks made with reference to the operation of one oscillator circuit will apply with equal force to the other, if postscript letters are changed accordingly.

To the end that firing pulses may be applied to the gate of thyristor 10a in accordance with the voltage appearing between control junction 17a and ground, the power circuit of a controllable conducting means 18a, which here takes the form of an NPN transistor, is connected between positive bus 14a and ground 15a through the exciting winding 19a of a feedback transformer 20a, a current limiting resistor 21a, and

24a, inverse parallel connected diodes 25a and 26a, ground and conductors 27a and 22a.

The operation of blocking oscillator 11A will now be described. Assuming transistor 18a is initially noneonducting, a current will flow from positive bus 14a through resistor 280, the base-emitter junction of transistor 18a and conductor 22a to ground. This base-emitter current initiates a flow of current from positive bus 14a through resistor 21a, exciting winding [9a, the collector-emitter power circuit of transistor 18a and conductor 22a to ground. The latter current induces a positive voltage across winding 19a. As a result of the voltage induced across exciting winding 19a, an induced voltage appears across feedback winding 23a which aids the current flowing in the base-emitter circuit of transistor 18a through resistor 28a. This further increases the collector-emitter conduction of transistor 180. Thus, the conduction of transistor 18a increases regeneratively to establish a firing pulse on output winding 16a.

After the foregoing activity has continued for a predetermined time, the core of feedback transformer 20a will saturate to terminate the feedback voltage induced across winding 23a. This termination decreases the conduction of transistor 18a. As a result, the polarity of the voltage across exciting winding 19a reverses as the inductance of the latter attempts to maintain theprevious value of current therethrough. Because of the reversal in the polarity of the voltage across winding 19a, the polarity of the induced voltage across winding 23a reverses to oppose the Current flowing in the base-emitter circuit of transistor 180 through resistor 28a. This further decreases the conduction of transistor 18a. Thus, the conduction of transistor 18a decreases regeneratively after the core of transformer a saturates.

After the energy stored in the magnetic field of exciting winding 19a is exhausted, pulse generating circuit 11A is restored to its initial condition. When this occurs, the circuit may begin another pulse generating cycle. In the present instance the time required for the magnetic field energy to exhaust itself is determined by the breakdown voltage of a zener diode 29a and the forwardvoltage drop of a diode 300. Thus,

. diodes 29a and 30a determine the operative frequency of firing pulse-generating circuit 1 1A.

In view of the foregoing, it is apparent that an'essential condition for the generation of firing pulses in firing pulsegenerating circuit "A is the presence of a positive voltage between control junction 17a and ground, which voltage must have a magnitude sufficient to establish the above-described initial base-emitter current in transistor" 18a. Thus, forcing control junction 17a to ground potential will prevent the generation of firing pulses in firing pulse-generating circuit 1 1A.

To the end that pulse-generating circuits 11A and 1113 may be energized alternately and severally, the switching circuit 12 is provided. This circuit comprises an astable, multistate switching'circuit which energizes first one and then the other of the firing pulse-generating circuits by establishing ground potential at first one and then the other of its output terminals 310 and 31b, these output terminals being connected to control junctions 17a and 17b, respectively.

When, for example, switch means 12 is in'a first operative state, terminal 31a attains a predetermined positive voltage with respect to ground and terminal 31b is substantially at ground potential. Under these conditions, pulse-generating circuit 11A can generate firing pulses and pulse-generating circuit 118 is deenergized. When, however, switch means 12 is in its second operative state, the latter voltage conditions reverse. At this time pulse-generating circuit llB can generate firing pulses and pulse-generating circuit "A is deenergized. Thus, switching means 12 energizes the firing pulse-generating circuits in a predetermined order by establishing a recurrent sequence of voltage conditions at its output terminals. Since the function of switching means 12 may be served by a number of electrical circuits including astable multivibrators, counters and the like which are well known to those skilled in the art, this portion of the circuit is shown in block form only.

The operation of switching means 12 in conjunction with firing pulse-generating circuits 11A and 11B can result in a generation of simultaneous firing pulses from firing pulsegenerating circuits 11A and 118. This will occur, for example, if both of the firing pulse-generating circuits are in the initial or firing pulse-generating portion of their operating cycle at the time when switching means 12 is changing states. This is because switch means 12 is unable to control the operation of the firing pulse-generating circuits while it is changing states. Thus, prior to the present invention, circuits of the abovedescribed type were subject to the simultaneous firing of two or more thyristors and the resultant serious fault conditions.

To the end that the above-described undesirable condition may be avoided, there is provided a start suppressing circuit which, in the present instance, takes the form of windings 13a and 13b. Winding 13a applies firing pulses produced by firing pulse-generating means 11A between ground and the control junction 17b of firing pulse-generating means 118. Similarly, winding 13b applies firing pulses produced by firing pulsegenerating circuit 118 between ground and the control juncfiring pulse suppresses the starting of the remaining firing pulse-generating circuit until the end of such'pulse. Thus, simultaneous firing pulses are eliminated. v I

When, for example, firing pulse-generating circuit 11A produces a firing pulse, a voltage-is induced across winding 13a positive on the top. This voltageforward biases diode 32a and initiates a current from conductor; 14a through resistor 28b, a conductor 33a, winding l3a,'and'diode 32a to ground. Because of the voltage drop which this current produces across resistor 28b, the potential of control" junction 17h becomes less positive. In the present embodiment this change in potential between ground (the emitter of transistor 18a) and control junction 1712 (the base of transistor 18a) is sufficient to cutoff transistor 18b. Thus, the generation of firing pulses from firing pulse-generating circuit [1B is not possible, during the time when a firing pulse is being produced by circuit 11A.

ln view of the foregoing, it is apparent that the circuitry of the invention utilizes the firing pulses produced by one firing pulse-generating circuit to disable the other firing pulseg'enerating circuit. Consequently, .whichcvcr firing pulsegenerating circuit is first to produce a firing pulse, during the time when switching means 12 is changing states, will continue to produce such firing pulse, the starting of the other firing pulse-generating circuit being suppressed until the end of this first pulse. Thus, simultaneous firing pulses from more than one firing pulse-generating circuit and the resultant simultaneous firing of more than one thyristor are eliminated.

While the foregoing description was made with reference to a thyristor control circuit having two firing pulse-generating circuits, it will be understood that circuitry of the invention may be utilized with thyristor control circuits having more than two thyristors and more than two firing pulse-generating circuits. The utilization of the control circuitry of the invention with thyristor control circuits having more than two firing pulse-generating circuits merely requires the connection of each start suppression winding such as 13a between ground and the control junctions of more than one other firing pulsegenerating circuit.

it will be understood that the embodiment shown herein is for illustrative purposes only and may be modified without departing from the spirit and scope of the appended claims;

lclaim: I

1. ha control circuit for providing firing pulses to each of a plurality of gate controlled-switchingelements, in combination, a source of DC voltage, a plurality of firing pulsegenerating circuits, means for connecting said firing pulsegenerating circuits across said DC source, each of said firing pulse-generating circuits including a control junction, start suppression means, means for connecting said start suppression means in pulse responsive relationship to respective firing pulse-generating circuits, means for connecting the start suppression means of each firing pulse generating circuit in deenergizing relationship to predetermined other firing pulsegenerating circuits, switching means having a plurality of outputs, means for connecting said switching means across said DC source, means for connecting the outputs of said switching means in control relationship to the control junctions of respective firing pulse-generating circuits. W v

2. in a control circuit for providing firing pulses to each of a plurality of gate controlled-switching elements, in combination, a source of voltage, a plurality of firing pulsegenerating circuits each having a control junction, means for connecting the firing pulse generating circuits across said DC source, the generation of firing pulses by each firing pulsegenerating circuit being controlled in accordance with the potential between one terminal of said DC source and the respective control junction, switching means having a plurality of outputs, means for connecting said switching means across said DC source, means for connecting the outputs of said switching means in energizing and deenergizing relationship to the control junctions of respective-firing pulse-generating circuits, start suppression means, means for connecting said start suppression means in pulse responsive relationship to respective firing pulse-generating circuits, means for connecting said start suppression means to the control junctions of predetermined other firing pulse-generating circuits in deenergizing relationship thereto, the generation of a firing pulse by any firing pulse-generating circuit serving to deenergize predetermined other firing'pulse-generating circuits during said firing pulse.

3. In a control circuit for providing firing pulses to each of a plurality of gate controlled-switching elements, in combination, a DC source; a plurality of oscillator circuits each including output means, controllable conducting means having a power circuit and a control circuit, means for connecting said controllable conducting means across said DC source in pulse generating relationship to respective output means; the generation of firing pulses by each of said oscillator circuits being controlled in accordance with the potential across the control circuits of respective controllable conducting means, means for alternately and severally applying 'an oscillation sustaining potential across the control circuits of respective controllable conducting means, start suppression means, means for connecting said start suppression means in pulse responsive relationship to the output means of respective oscillator circuits, means for connecting the start suppression means of each oscillator circuit in voltage control relationship to the control circuits of the controllable conducting means of predetermined other oscillator circuits, the appearance of a firing pulse at the output means of any oscillator circuit serv ing to prevent the application of an oscillation sustaining potential across the control circuits of the controllable conducting means of predetermined other oscillator circuits.

4. in a circuit for providing firing pulses to each of a plurality of gate controlled-switching elements, in combination, a

DC source; a plurality of oscillator circuits each including controllable conducting means having a power circuit and a control electrode, means for connecting said control electrodes to one terminal of said DC source, output means; means for connecting said output means in series with the power circuits of respective controllable conducting means, means for connecting said DC source in energizing relationship to respective oscillator circuits, each of said oscillator circuits being adapted to provide a succession of firing pulses when an oscillation sustaining voltage appears between the respective control electrode and one tenninal of said DC source, means for alternately and severally applying oscillation sustaining voltages to the control electrodes of said oscillator circuits, start suppression means, means for disposing said start suppression means in pulse responsive relationship to the output means of respective oscillator circuits, means for connecting a first end of said start suppression means to one terminal of said DC source, means for connecting the other end of said start suppression means in oscillation sustaining voltage controlling relationship to the control electrodes of the controllable conducting means of predetermined other firing pulse-generating circuits.

5. In a circuit for providing firing pulses to each of a plurality of gate controlled-switching devices, in combination, a DC source; a plurality of firing pulse-generating circuits each including a controllable conducting means having a power circuit and a control electrode, means for connecting said con trol electrode to one terminal of said DC source, transformer means, primary winding means on said transformer means; means for connecting said primary winding means in closed circuit relationship with the power circuits of respective controllable conducting means and said DC source, output winding means on each of said transformers, the firing pulse 1 generating activity of said tiring pulse-generating circuits being controlled in accordance with the presence of an enabling voltage between the respective control electrodes and one terminal of said DC source, means for alternately and severally applying enabling voltages to respective control electrodes of the controllable conducting means of said firing pulseenerating circuits, start su pression winding means on each 0 said transformers, means. or connecting said start suppression winding means in pulse responsive relationship to the transformer means of respective firing pulse-generating circuits, means for connecting the start suppression winding means of one firing pulse-generating circuit between one terminal of said DC source and the control electrodes of the controllable conducting means of predetermined other firing pulse-generating circuits, the voltage induced upon any of said start suppression winding means as a result of the generation of a firing pulse by respective firing pulse-generating circuits serving to remove said enabling voltage from the control electrodes of said predetermined other firing pulse-generating circuits.

6. in a circuit for providing firing pulses to each ofa plurality of gate controlled-switching devices, in combination, a DC source; a plurality of firing pulse-generating circuits each including a transistor having a power circuit and a control circuit, means for connecting said control circuit to one terminal of said DC source, transformer means having exciting winding means, feedback winding means, output winding means and start suppression winding means, means for connecting said exciting winding means and the power circuit of said transistor in closed circuit relationship with said DC source, means for connecting said feedback winding means in closed circuit relationship with the control circuit of said transistor; means for connecting said output winding means to the gates of respective gate controlled-switching devices, the firing pulse generating activity of said firing pulse-generating circuits being controlled in accordance with the presence of an enabling voltage between one terminal of said DC source and the respective control circuits, means for alternately and severally applying enabling voltages to said respective control circuits, means for connecting the start suppression winding means of each firing pulse-generating circuit between one terminal of said DC source and the control circuits of the transistors of predetermined other firing pulse-generating circuits, the voltage induced across any start suppression winding means as a result of the generation of firing pulses by the respective firing pulse-generating circuit serving to remove said enabling voltage from the control circuits of said predetermined other firing pulse-generating circuits. 

1. In a control circuit for providing firing pulses to each of a plurality of gate controlled-switching elements, in combination, a source of DC voltage, a plurality of firing pulse-generating circuits, means for connecting said firing pulse-generating circuits across said DC source, each of said firing pulsegenerating circuits including a control junction, start suppression means, means for connecting said start suppression means in pulse responsive relationship to respective firing pulse-generating circuits, means for connecting the start suppression means of each firing pulse generating circuit in deenergizing relationship to predetermined other firing pulsegenerating circuits, switching means having a plurality of outputs, means for connecting said switching means across said DC source, means for connecting the outputs of said switching means in control relationship to the control junctions of respective firing pulse-generating circuits.
 2. In a control circuit for providing firing pulses to each of a plurality of gate controlled-switching elements, in combination, a source of DC voltage, a plurality of firing pulse-generating circuits each having a control junction, means for connecting the firing pulse-generating circuits across said DC source, the generation of firing pulses by each firing pulse-generating circuit being controlled in accordance with the potential between one terminal of said DC source and the respective control junction, switching means having a plurality of outputs, means for connecting said switching means across said DC source, means for connecting the outputs of said switching means in energizing and deenergizing relationship to the control junctions of respective firing pulse-generating circuits, start suppression means, means for connecting said start suppression means in pulse responsive relationship to respective firing pulse-generating circuits, means for connecting said start suppression means to the control junctions of predetermined other firing pulse-generating circuits in deenergizing relationship thereto, the generation of a firing pulse by any firing pulse-generating circuit serving to deenergize predetermined other firing pulse-generating circuits during said firing pulse.
 3. In a control circuit for providing firing pulses to each of a plurality of gate controlled-switching elements, in combination, a DC source; a plurality of oscillator circuits each including output means, controllable conducting means having a power circuit and a control circuit, means for connecting said controllable conducting means across said DC source in pulse generating relationship to respective output means; the generation of firing pulses by each of said oscillator circuits being controlled in accordance with the potential across the control circuits of respective controllable conducting means, means for alternately and severally applying an oscillation sustaining potential across the control circuits of respective controllable conducting means, start suppression means, means for connecting said start suppression means in pulse responsive relationship to the output means of respective oscillator circuits, means for connecting the start suppression means of each oscillator circuit in voltage control relationship to the control circuits of the controllAble conducting means of predetermined other oscillator circuits, the appearance of a firing pulse at the output means of any oscillator circuit serving to prevent the application of an oscillation sustaining potential across the control circuits of the controllable conducting means of predetermined other oscillator circuits.
 4. In a circuit for providing firing pulses to each of a plurality of gate controlled-switching elements, in combination, a DC source; a plurality of oscillator circuits each including controllable conducting means having a power circuit and a control electrode, means for connecting said control electrodes to one terminal of said DC source, output means; means for connecting said output means in series with the power circuits of respective controllable conducting means, means for connecting said DC source in energizing relationship to respective oscillator circuits, each of said oscillator circuits being adapted to provide a succession of firing pulses when an oscillation sustaining voltage appears between the respective control electrode and one terminal of said DC source, means for alternately and severally applying oscillation sustaining voltages to the control electrodes of said oscillator circuits, start suppression means, means for disposing said start suppression means in pulse responsive relationship to the output means of respective oscillator circuits, means for connecting a first end of said start suppression means to one terminal of said DC source, means for connecting the other end of said start suppression means in oscillation sustaining voltage controlling relationship to the control electrodes of the controllable conducting means of predetermined other firing pulse-generating circuits.
 5. In a circuit for providing firing pulses to each of a plurality of gate controlled-switching devices, in combination, a DC source; a plurality of firing pulse-generating circuits each including a controllable conducting means having a power circuit and a control electrode, means for connecting said control electrode to one terminal of said DC source, transformer means, primary winding means on said transformer means; means for connecting said primary winding means in closed circuit relationship with the power circuits of respective controllable conducting means and said DC source, output winding means on each of said transformers, the firing pulse generating activity of said firing pulse-generating circuits being controlled in accordance with the presence of an enabling voltage between the respective control electrodes and one terminal of said DC source, means for alternately and severally applying enabling voltages to respective control electrodes of the controllable conducting means of said firing pulse-generating circuits, start suppression winding means on each of said transformers, means for connecting said start suppression winding means in pulse responsive relationship to the transformer means of respective firing pulse-generating circuits, means for connecting the start suppression winding means of one firing pulse-generating circuit between one terminal of said DC source and the control electrodes of the controllable conducting means of predetermined other firing pulse-generating circuits, the voltage induced upon any of said start suppression winding means as a result of the generation of a firing pulse by respective firing pulse-generating circuits serving to remove said enabling voltage from the control electrodes of said predetermined other firing pulse-generating circuits.
 6. In a circuit for providing firing pulses to each of a plurality of gate controlled-switching devices, in combination, a DC source; a plurality of firing pulse-generating circuits each including a transistor having a power circuit and a control circuit, means for connecting said control circuit to one terminal of said DC source, transformer means having exciting winding means, feedback winding means, output winding means and start supprEssion winding means, means for connecting said exciting winding means and the power circuit of said transistor in closed circuit relationship with said DC source, means for connecting said feedback winding means in closed circuit relationship with the control circuit of said transistor; means for connecting said output winding means to the gates of respective gate controlled-switching devices, the firing pulse generating activity of said firing pulse-generating circuits being controlled in accordance with the presence of an enabling voltage between one terminal of said DC source and the respective control circuits, means for alternately and severally applying enabling voltages to said respective control circuits, means for connecting the start suppression winding means of each firing pulse-generating circuit between one terminal of said DC source and the control circuits of the transistors of predetermined other firing pulse-generating circuits, the voltage induced across any start suppression winding means as a result of the generation of firing pulses by the respective firing pulse-generating circuit serving to remove said enabling voltage from the control circuits of said predetermined other firing pulse-generating circuits. 